If you are on iOS 7 you can check your cellular data usage by app. Go to Settings > Cellular. There will be a list of your apps at the bottom along with how much data they've used that period.
The data/period is reset at the bottom of the screen, where it will also tell you when that last reset was.
You can turn ON/OFF access to cellular data for each app, so you can trim out the unnecessary stuff if needed. You can also turn off Cellular data in Settings > iTunes & App Store. This will prevent you from wasting your cellular plan on app updates and other automatic content downloads. The iPad will wait until you have a wi-fi connection to do them.
Of course, this keeps you from buying new stuff when off wi-fi as well.
There are some third party apps/services in the App Store that will monitor your wi-fi data usage per app. They work by rerouting your connection through their servers, so they can tell what got sent/received where. It's a bit invasive, and you're depending on their integrity not to gather/sell your information, but probably the only way to do this from a stock iPad. It is probably not worth it unless you both really need to know, and your wi-fi ISP has a limited data plan that you are in danger of exceeding.